Many commercial buildings as well as homes are heated by forced hot air furnaces. These furnaces typically include a boiler using an oil or gas-fired burner, a heat exchanger, and an air blower or fan. The heat exchanger typically comprises a plurality of passageways through which steam from the boiler flows. The fan is mounted next to the heat exchanger such that cool air may be forced past the heat exchanger and heated. The fan, which is typically powered by an electric motor, also moves the heated air through the building or home via an arrangement of ducts leading to the various rooms. An electric thermostat operably connected to the burner and the fan is often used to control the furnace. The thermostat switches the furnace on (e.g., activates the burner and the fan) whenever the temperature at the thermostat falls below a preselected level. Operation of the furnace brings warm air into the home. When the temperature at the thermostat exceeds another preselected level, the thermostat shuts the furnace off, thereby suspending the flow of heated air.
One of the main disadvantages of such furnaces is their dependence on electricity. As described above, such furnaces generally include an electrically powered fan to move air past the heat exchanger and through the building or home being heated. In addition, electric power is often used to operate the thermostat and to control the burner. For a 100,000 Btu/hour residential forced hot air furnace, for example, the electric power requirement is typically between 0.5 to 1.0 kilowatts. Furthermore, if the electricity delivered to the furnace is interrupted for whatever reason, the furnace is rendered inoperable. That is, without electric power, the thermostat, the burner and the motor that drives the fan will not work, thereby stopping the flow of warm air to the space(s) being heated.
Since electric power is often lost during blizzard conditions or other cold weather storms, the concomitant loss of the furnace's heating ability during periods of otherwise large demands for heat can have serious consequences. For example, if the power is disrupted for any length of time, the building or home can become so cold as to be uninhabitable. In addition, the temperature in the building or home may fall below freezing, causing water pipes to burst. The resulting water damage can be substantial.
In addition to forced hot air heating systems, many older homes and buildings use a steam heating system. With these systems, steam from the boiler is distributed to a series of radiators disposed throughout the building using its own pressure energy. However, the cost of steam distribution systems is relatively high compared to modern forced hot air heating systems. Additionally, forced hot air systems can be easily modified to provide both heating and air conditioning. Accordingly, for reasons of economy and convenience, forced hot air is now the most widely used heating system, despite the disadvantage described above.
One heating system that combined aspects of both steam heating and forced hot air heating was the SelecTemp system from Iron Fireman Manufacturing Company. As shown in the corresponding Application, Installation and Service Manual, the SelecTemp system, which has not been in production for many years, included a central steam boiler that provided steam to each of the rooms being heated. A mini heat exchanger and fan combination was located in each room. Steam from the boiler was delivered to the heat exchanger and to a small turbine that operated the fan. The steam was thus utilized to power the fan and to generate the heat that was subsequently forced into the room by the small fan. Condensate from each heat exchanger and fan combination drained back to the boiler in a return piping system separate from the steam supply piping. The condensate was collected in a common sump at atmospheric pressure and was returned to the central boiler by a pump. The pump was either powered electrically or by the steam produced from the boiler (e.g., by another small turbine with its own steam supply line). Although the SelecTemp system, including the steam-powered return pump, was not dependent on electricity, it was disadvantageous for several reasons.
First, the configuration of multiple heat exchangers in separate rooms and a common sump precluded the system from being operated or producing heat at more efficient vacuum steam temperatures and pressures. That is, in order to equalize the pressure at each heat exchanger and thereby ensure the return of condensate to the sump, the SelecTemp system specifically required that the condensate pump be vented to atmospheric pressure. By venting the condensate pump to atmospheric pressure, the heat exchangers were forced to operate at or somewhat above atmospheric pressure. For boilers manufactured in accordance with the American Society of Mechanical Engineers (ASME) Pressure Vessel and Boiler Code (Section IV, Heating Boilers), moreover, the maximum steam pressure that may be generated by such boilers is about 10 psig. Accordingly, the corresponding pressure ratios at the turbines was relatively low, and thus the available pressure energy that could be extracted to drive the fans was extremely limited. Indeed, in the SelecTemp system, the amount of mechanical power that could be extracted from the steam to power the fan is only about half that which would be available if the heat exchanger were capable of being operated at vacuum steam pressure.
Another major disadvantage of the SelecTemp system is that the boiler must be maintained at or near its full working pressure and temperature in order for the system to provide heat. That is, to achieve condensation at the heat exchangers (which operated at or somewhat above atmospheric pressure) and thereby heat the rooms, the steam being supplied to the heat exchangers needed to be at least 212 degrees Fahrenheit and positive pressure (relative to atmospheric). Once the boiler stopped producing positive pressure steam at 212 degrees Fahrenheit, the heat transfer process ceased. Accordingly, the energy used to bring the boiler to its operating pressure and temperature, which was often substantial due to the large mass of most cast iron boilers, was not available for heating the building. A significant amount of heat energy supplied by the burner was thus never realized.
The SelecTemp system was also relatively complex and expensive to manufacture, install and maintain. In particular, the system included a separate heat exchanger, fan, turbine and control valve in each room within the space being heated. These numerous working parts, which were dispersed throughout the building, added to the system's complexity and cost. The SelecTemp system further required that steam be provided (typically by 1/4 inch copper tubing) to each room, resulting in significant thermodynamic losses. In addition, in order to drain accumulating condensate from these lines, steam traps were required, which were prone to leakage, thereby causing additional problems.
The SelecTemp system also did not lend itself to easy installation in existing homes as a replacement furnace, especially for forced hot-air furnaces. That is, the system was generally a completely new installation requiring substantial construction work to provide steam pipes running from the boiler to the heat exchanger/fan combination in each room. Thus, one could not readily convert an existing, conventional forced hot-air furnace to the SelecTemp system.